test(build): reject invalid client handler names

Drive AppBuilder through a rooted client module with a non-Rust handler name and assert the exact InvalidData diagnostic, closing the public invalid-value mutation seam.

req: build/004

req: client_local/011

req: diagnostics/004

req: test/021

hemx-build/src/lib.rs

@@ -2512,7 +2512,7 @@ mod tests {
     }
 
     #[test]
-    fn app_builder_returns_output_errors_instead_of_panicking() {
+    fn app_builder_propagates_output_and_client_contract_errors_without_panicking() {
         let root = std::env::temp_dir().join(format!(
             "hemx-build-error-{}-{}",
             std::process::id(),
@@ -2552,8 +2552,29 @@ mod tests {
             result.unwrap().unwrap_err().to_string(),
             "client-local handlers require one data-hemx-client-module on a hemx root"
         );
+
+        let invalid_handler = templates.join("invalid_handler.heml");
+        std::fs::write(
+            &invalid_handler,
+            r#"<main data-hemx-root="app" data-hemx-client-module="/app.js"><button data-hemx-client="save-item">Save</button></main>"#,
+        )
+        .unwrap();
+        std::fs::remove_file(templates.join("client.heml")).unwrap();
+        let error = app()
+            .template_dir(&templates)
+            .out_dir(&invalid_out)
+            .run()
+            .unwrap_err();
+        assert_eq!(error.kind(), io::ErrorKind::InvalidData);
+        assert_eq!(
+            error.to_string(),
+            format!(
+                "{}: invalid data-hemx-client value `save-item`; expected a Rust handler identifier",
+                invalid_handler.display()
+            )
+        );
         let _ = std::fs::remove_dir_all(root);
-        // test req: build/004
+        // test req: build/004 req: client_local/011 req: diagnostics/004
     }
 
     #[test]